Light emitting diodes (LEDs) can potentially replace incandescent, fluorescent and arc lamp sources for many lighting applications. However, one issue that currently restricts LED deployment is low light output efficiency. The light output efficiency of an LED is determined both by the internal quantum efficiency of converting electrical energy into photons and by the efficiency of light extraction from the device.
The light extraction efficiency of an LED die is strongly dependent on the refractive index of the LED relative to its surroundings, to the shape of the die, and to the absorption coefficient alpha (α) of the semiconductor layers. For example, increasing the refractive index of the LED relative to its surroundings will decrease the light extraction efficiency. An LED die with flat external sides and right angles to its shape will have lower light extraction efficiency than an LED with beveled sides. Increasing the absorption coefficient alpha of the semiconductor layers will decrease the light extraction efficiency.
Solid-state LEDs are generally constructed from semiconductor materials that have a high refractive index (n>2) and high light absorption coefficients. For example, GaN, InGaN and AlGaN light emitting materials used in constructing ultraviolet, blue, cyan and green LEDs dies have a refractive index of approximately 2.5 and absorption coefficients α of 10 cm−1 to 200 cm−1 or thereabouts in the light emitting region and the heavily-doped semiconductor layers of the LED die. The absorption coefficient of the GaN-based semiconductor layers is sometimes difficult to determine accurately because of light scattering that is also present in the materials. Both the high refractive index and the high absorption inhibit light extraction from the device.
If the LED die has a refractive index ndie, has flat external surfaces, and furthermore is in contact with an external material such as air that has a refractive index next, only light that has an angle less than the critical angle will exit from the die. The remainder of the light will undergo total internal reflection at the inside surfaces of the die and remain inside the die. The critical angle θc inside the die is given byθc=arcsin(next/ndie),   [Equation 1]where θc is measured relative to a direction perpendicular to the LED surface. For example, if the external material is air with a refractive index next of 1.00 and the refractive index ndie is 2.5, the critical angle is approximately 24 degrees. Only light having incident angles between zero and 24 degrees will be extracted. The majority of the light generated by the active region of the LED will strike the surface interface at angles between 24 degrees and 90 degrees and will undergo total internal reflection. The light that is totally internally reflected will remain in the die until it is either absorbed or until it reaches another surface that may allow the light to exit.
The amount T of light that is transmitted through an optical pathlength L of an LED die having an absorption coefficient α is given byT=e−αL.  [Equation 2]If one wishes to keep the absorption less than 20% or conversely keep the transmission T greater than 80%, for example, then the quantity αL in Equation 2 should be about 0.2 or less. If α=50 cm−1, for example, then L should be less than about 0.004 centimeters or 40 microns in order to keep the absorption less than about 20%. Since many LED die materials have semiconductor layers with absorption coefficients on the order of 10 cm−1 to 200 cm−1 and since many LED dies have lateral dimensions of 300 microns or larger, a large fraction of the light generated by the die can be absorbed inside the die before it can be extracted.
Many ideas have been proposed for increasing the light extraction efficiency of LEDs. These ideas include forming angled (beveled) edges on the die, adding non-planar surface structures to the die, roughening at least one surface of the die, and encapsulating the die in a material that has a refractive index intermediate between ndie and the refractive index of air. For example, U.S. Patent Application Ser. No. 20020123164 discloses using a series of grooves or holes in the substrate portion of the die as light extracting elements. The substrate portion of the die can be, for example, the silicon carbide or sapphire substrate portion of a die onto which the GaN-based semiconductor layers are fabricated. However, in U.S. Patent Application Ser. No. 20020123164 the grooves or holes do not extend into the semiconductor layers. If the substrate is sapphire, which has a lower index of refraction than GaN, much of the light can still travel relatively long distances within the GaN-based semiconductor layers before reaching the edge of the die.
U.S. Pat. No. 6,410,942 discloses the formation of arrays of micro-LEDs on a common substrate to reduce the distance that emitted light must travel in the LEDs before exiting the LEDs. Micro-LEDs are formed by etching trenches or holes through the semiconductor layers that are fabricated on the substrate. Trenches are normally etched between LEDs on an array to electrically isolate the LEDs.
However, in U.S. Pat. No. 6,410,942 the substrate remains as part of the micro-LED structure and is not removed. The substrate adds to the thickness of the LED die and can reduce the overall light extraction efficiency of the array. Even if light is efficiently extracted from one micro-LED, it can enter the substrate, undergo total internal reflection from the opposing surface of the substrate, and be reflected back into adjacent micro-LEDs where it may be absorbed.
U.S. Pat. No. 6,410,942 and other patents on light extraction do not disclose how to make LEDs or arrays of micro-LEDs that are highly reflective. Little thought is given to how well the LEDs reflect light incident from other light sources or nearby reflecting surfaces. However, the reflectivity of an LED to incident light is critically important for applications where some of the light emitted into the external environment by the LED is reflected or recycled back to the LED. For example, U.S. patent application Ser. No. 10/445,136 by Zimmerman and Beeson and U.S. patent application Ser. No. 10/814,043 by Beeson and Zimmerman, both of which are herein incorporated by reference, propose that light recycling can be utilized to construct enhanced brightness LED optical illumination systems. In the two above-mentioned patent applications, the LEDs are located inside light reflecting cavities or light recycling envelopes and light is reflected off the surfaces of the LEDs in order to achieve the enhanced brightness. In a second example, Steranka et al in U.S. Pat. No. 6,730,940 disclose an enhanced brightness light emitting device spot emitter that also requires LEDs that have high reflectivity. Thirdly, if a light source is comprised of both an LED and a phosphor that converts at least a part of the LED emitted light into another wavelength, the phosphor can reflect some of the emitted light back to the LED. If the LED has poor reflectivity, some of the reflected light will be absorbed by the LED and reduce the overall efficiency of the light source.
Increasing the density of light extracting elements by decreasing the size of micro-LEDs in U.S. Pat. No. 6,410,942 may increase the light extraction efficiency of a single micro-LED, but can also decrease the reflectivity of the micro-LED to incident light. The same structures that extract light from the LED die also cause light that is incident onto the die to be injected into the high-loss semiconductor layers and to be transported for relatively long distances within the layers. Light that travels for long distances within the semiconductor layers is strongly absorbed and only a small portion may escape from the die as reflected light. In one embodiment of U.S. Pat. No. 6,410,942, the micro-LEDs are circular with a diameter of 1 to 50 microns. In another embodiment, the micro-LEDs are formed by etching holes through the semiconductor layers resulting in micro-LEDs with a preferred width between 1 and 30 microns. Micro-LEDs with such a high density of light extracting elements can have reduced reflectivity for incident light.
In comparison to surfaces that have a high density of light extracting elements, smooth LED surfaces that do not have light extracting elements have poor light extraction efficiency but can be good light reflectors. Light that is incident on the surface will be refracted to smaller angles (less than the critical angle in Equation 1) inside the LED die, will travel directly across the thin semiconductor layers, will be reflected by a back mirror surface, will travel directly across the semiconductor layers a second time and then exit the LED die surface as reflected light. In such cases, the incident light is not trapped in the semiconductor layers by total internal reflection and does not necessarily undergo excessive absorption.
In general, LED light extraction efficiency and reflectivity are inversely related. Improving one of the two quantities tends to degrade the other quantity.
Another reason for the low reflectivity of many current LED designs is that the LED die may include a substrate that absorbs a significant amount of light. For example, GaN-based LEDs that have a silicon carbide substrate are usually poor light reflectors with an overall reflectivity of less than 60%. One reason for the low reflectivity is that both the GaN semiconductor layers and the silicon carbide absorb part of the incident light.
An additional reason for the low reflectivity of many current LED designs is that external structures on the LEDs, including the top metal electrodes, metal wire bonds and sub-mounts to which the LEDs are attached, are not designed with high reflectivity in mind. For example, the top metal electrodes and wire bonds on many LEDs contain materials such as gold that have relatively poor reflectivity. Reflectivity numbers on the order of 50% are common.
Present LED designs usually have either relatively low optical reflectivity (less than 60%, for example) or have high reflectivity combined with low light extraction efficiency (for example, less than 20%). For enhanced brightness illumination systems utilizing light recycling and for systems utilizing phosphors, it would be desirable to have LEDs that exhibit both high reflectivity and high light extraction.